Recently, as LED is variously produced due to the brilliant development of semiconductor technology, the trend is that LED whose power consumption needed to obtain the same illuminance as a general light bulb is no more than about 1/10 of it is applied to lamps for signboard, illumination and traffic lights
However, a separate power supply device is needed to drive the lighting that uses LED.
That is, the lighting that uses a general bulb in the art does not need a separate power supply device since it uses an alternating AC power supply of 220V. However, since the LED uses direct DC power supply, SMPS should be installed in order to rectify the commercial alternating power supply of 220V into a direct voltage and convert the rectified direct voltage into a direct voltage having a magnitude required to turn on the lighting diode.
However, when a general SMPS scheme is adopted, a power-factor controller is used in the prior stage of the SMPS, so that the circuit itself not only is very complicated but also the device becomes large in volume since the circuit uses an inductor, an electrolytic condenser and so on.
Further, since the electrolytic condenser has the lifetime that is no more than 2 or 3 years, there is a problem that it is not suitable to the LED lamp that should have a long lifetime. Further, there is a problem that the circuit becomes more complicated by adding a noise filter to prevent a ripple voltage, which may cause an electro-magnetic interference EMI occurring when switching is made.
Additionally, in the ballast circuit for an LED lamp in the art, a DC constant-voltage circuit is constructed normally using SMPS, and a current limit resistor is used in each LED module to drive the LED stably.
However, since the SMPS is constructed of very precise and complicated circuits and should be switched at a high speed theoretically, there were problems that the SMPS might generate heat and noise easily, might be expensive and it was not easy to expect a high reliability and a long lifetime.
Meanwhile, while a simplified power supply device to drive simple electronic equipment by dropping voltage using resistors or capacitors without transformer in a commercial power supply is widely known in the name of so called “transless power supply” and partially used in door sensor lamp, remote control receiver and appliances controller, it is not possible to use the simplified power supply device as a stable power supply even though generally a constant-voltage diode or the like is additionally used since voltage fluctuations caused by the changed loads are severe, only a few current is flowed, and there is no countermeasure for kinds of noises coming in from the power supply.
Merely, when current is limited from the commercial power supply using a capacitor having a suitable capacitance and LEDs are serially connected, applied voltage has a characteristic that it is stably applied by a potential difference of PN junction x the number of LEDs in view of the characteristic of LED that is a kind of diode, and so there have been studies to utilize the characteristic. However, when the LED for illumination that is very sensitive to electrical shock and requires a long lifetime is directly connected to a commercial power supply without any protective circuit, the LED is broken due to surge voltage occurring when the power supply is applied and high frequency component noise that may be included in the power supply. Therefore, there is no industrial applicability although it may be possible experimentally.
Especially, when the LED is directly connected to the power supply by way of a capacitor and a bridge diode, it was known that the LED could not be protected effectively even though kinds of surge voltage control devices developed recently (for example, barrister, TVS diode, surge protector and so on) were applied according to experiment.
The ballast circuit for an LED lamp in the art that the applicant has suggested to solve the problems described above is connected to a current limit unit that is constructed of a capacitor C1 having a suitable capacitance correspondingly to characteristic of the alternating power supply AC 220V and a resistor R1 connected in parallel with the capacitor in order to charge and discharge on an end of the alternating power supply AC 220V in period of 60 Hz and control LED use current as illustrated in FIG. 1.
An output stage of the current limit unit is connected to the bridge diode BD that rectifies alternating voltage into direct voltage, the bridge diode BD is sequentially connected to a smoothing capacitor C2, a discharge resistor R2, and a surge absorber formed of a capacitor C3 and a resistor R3 that are serially connected with each other between both ends of the bridge diode BD, and an output end of the bridge diode BD is serially connected to the LED protection resistor R4.
Further, a positive (+) terminal of the bridge diode BD is connected to LED arrays (DA1, DA2, . . . DAn) in which plural forward LEDs are serially connected with one another, and a negative (−) terminal of the bridge diode BD is connected to LED arrays (DA11, DA12, . . . DA1n) in which plural reverse LEDs are serially connected with one another.
However, while the smoothing capacitor C2 and discharge resistor R2 are additionally connected to cut off high frequency noise components coming in from the power supply and provide current to the LED arrays (DA1, DA2, . . . DAn) and LED arrays (DA11, DA12, . . . DA1n) stably in the ballast circuit for a LED lamp in the art illustrated in FIG. 1, since there are concerns that rush current flows in the capacitor C1 of the power supply and the smoothing capacitor C2 having a high capacitance and reverse voltage occurs when the power supply is applied and this shock may negatively affect lifetime of the LED arrays (DA1, DA2, . . . DAn) and LED arrays (DA11, DA12, . . . DA1n), a current limit resistor R4 is connected to the prior stage of the LED arrays (DA1, DA2, . . . DAn) in order to alleviate such phenomenon.
However, since LED drive current always flows in the current limit resistor R4, there were problems that heat occurred and energy loss also occurred.